Another of New Jersey's Priceless Resources - Studying the Delaware Bay

By Jennifer Pyle
Assistant Biologist
July 24, 2009

INTRODUCTION - HISTORY OF FISHERIES SURVEYS

The Delaware Estuary is New Jersey's largest estuary system. The Delaware estuary is a semi-enclosed body of water where freshwater from the Delaware River mixes with salt water from the Delaware Bay. It serves as nursery areas, spawning and feeding grounds, and migratory routes for many recreational and commercial fish.

Bureau of Marine Fisheries biologists within the New Jersey DEP's Division of Fish & Wildlife conduct several surveys each year to study the status of species populations within the estuary. One of these surveys is the Delaware Bay Finfish Trawl Survey.

In 1991, the Division began a Delaware Bay finfish trawl survey of juvenile finfish species to develop indices for comparing the relative annual abundance of selected stocks. The survey was designed to complement a similar effort being conducted on the western side of Delaware Bay by the State of Delaware's Division of Fish and Wildlife and sampling stations were set up within the shallow, near shore waters on the New Jersey side of the bay. Data collected allows biologists to develop relative abundance estimates and length frequencies of estuarine dependent finfish necessary for predicting future fishery trends and harvest potential. (See New Jersey's Priceless Resource - Studying the Delaware River for the report on a related survey.)

Click on the links below for more Delaware Bay Finfish Trawl Survey information:

The number of stations and their locations have varied over the course of the survey. Currently, there are eleven sampling stations located on shoals near the shoreline, extending from Villas in Cape May to the Cohansey River of Cumberland County. These near-shore stations have typically provided greater yields and more species diversity than sampling attempts in deeper waters.

Sampling
is performed with a 42-foot research vessel, the R/V Zephyrus.
As was the case with the number of stations, the months sampled
have also varied. During the survey, samples are collected at
each location once a month from April to October. Single ten-minute
tows are conducted against the tide at each station.

All
species collected are identified, counted and measured. If
counts are high, 50 individual lengths are randomly selected
and recorded. For finfish, from 1991 to 1999, total lengths
(tip of nose to end of tail) were measured. Since 2000, fork
lengths (tip of nose to inside fork of tail) have been recorded
for all species with a forked tail. For species with no forked
tail, such as Atlantic croaker, a total length is measured.

The net used is a 16-foot otter trawl. The top of the net
opening is buoyed with fish net floats. The bottom of the
net's mouth is weighted with a chain so that the net can be
pulled along the floor of the bay. There is a wooden "door"
that acts as a spreader off each end of the net. They ensure
that the net stays open while fishing. The doors are attached
to the chain from the bottom of the net and nylon towline
from the top of the net. The cod end is made of small mesh
so that the fish will be retained in this section.

The Division has hauled 1,322 tows and caught 349,873 fish for an average of 264 fish per tow since the survey's inception in 1991. A total of 89 different species have been identified with the five most abundant being bay anchovy, Atlantic croaker, weakfish, blue crab and Atlantic herring.

*NOTE: The number of stations, station locations and months sampled from 1991 to 1996 were inconsistent due to personnel constraints and weather conditions. This inconsistency will have an affect on the number of individuals collected and on the annual relative abundance indices for those years, especially for finfish that utilize the Delaware Bay on a seasonal basis.

Water
quality levels are recorded at each station. These parameters include
salinity, water temperature and dissolved oxygen
(DO). Measurements were taken at the bottom of the water column at
each station. They were recorded using a YSI brand dissolved oxygen
meter. The YSI meter was not functional during the 2007 survey period,
therefore there is no water quality data for 2007.

In this survey, there are 4 stations that are considered the "upper"
bay (area north of Egg Island Point), and 7 stations considered
"lower" bay (area south of Egg Island Point). Since the upper bay
is farther from the mouth of the ocean, the salinity in the upper
bay is lower than the salinity in the lower bay. When the water
quality measurements taken during this survey are examined by area,
there is only a slight difference in their averages. Therefore it
makes sense to look at this region as a whole.

Salinity is a measurement of the amount of salt in a body
of water. In the Delaware Bay trawl survey, it is measured in parts
per thousand (ppt) which is the measure of grams of salt per liter
of water. There are many factors which influence salinity. Freshwater
run-off from rain storms lowers levels, while droughts lead to an
influx of saltwater from the ocean. Survey averages have ranged
from 14.98 ppt in 2000 to 20.24 ppt in 2002. The lowest salinity
recorded was at station 12 in 2005, and had a reading of 2.0 ppt.
This station is located farthest from the ocean's saltwater influx.
The highest salinity recorded was at station 60 in 2006, and had
a reading of 30.7 ppt. Opposite of station 12, station 60 is located
closest to the ocean. Overall, trends show that the salinity of
the Delaware Bay has remained fairly constant through the years.Salinity
Graph (pdf, 7kb)

Water temperature is also affected by many factors, including
water depth, tides, and the weather. In this survey, temperatures
are measured as degrees Celsius (C). On average, the water temperature
during this survey, has remained consistent over the years. Average
temperatures range from 19.61 degrees C in 1997, to 23.32 degrees
C in 2005. The lowest recorded water temperature was 8.3 degrees
C at station 60 in 2003, and the highest water temperature was 31
degrees C at stations 39, 54 and 60 in 1993.Water
Temperature Graph (pdf, 10kb)

Dissolved oxygen (DO) is a measure of the amount of gaseous
oxygen that is dissolved in a body of water. It is measured in milligrams
per liter (mg/L). Oxygen gets into the water from the surrounding
air. This can occur by various ways, including: tides, run-off,
boat traffic, wind or storms. Trends show hardly any change in the
overall average of DO in the Delaware Bay. The survey averages ranged
from 5.42 mg/L in 1999 to 8.13 mg/L in 2000. The lowest recorded
DO was 2.0 at stations 47, 54 and 60 during 1999 and 2004. The highest
recording was 13.0 at station 54 in both 1994 and 1996..Dissolved
Oxygen Graph (pdf, 10kb)

Surveys like this are just the beginning of the stock assessment process for many species. For example, data from this and similar surveys in other states have reflected an increase in the striped bass population along the entire East Coast. They have also shown a decrease within the bay anchovy population.

As with any ecosystem, data collected from the Delaware Estuary surveys show fluctuations among all species. There are constant changes in the size of fish populations due to many environmental factors. While it is not always completely certain why any species population decreases or increases, the Division is planning future research to examine these variations.

Fishery surveys, such as the Delaware Bay Finfish Trawl, are important for ecosystem management. They provide biologists with information relative to the annual abundance and population trends of many species. Without these surveys, biologists would not be aware of the population increases or decreases that predict future fishery management needs. Being conscious of these fluctuations is important when creating recreational and commercial fishing regulations.